Do not throw exception when adding relationships to external elements

[goneall]

There are some examples where the SPDX file expresses a relationship FROM an external element.

Since these are properties of an external element and properties can not be stored if the element is external to the document, this call fails.

We could possibly ignore this error - although the information would be lost.

It is not clear if this should be solved or remain as an error and fix the examples.

[goneall]

Based on comments from @nishakm in spdx/spdx-examples#119, I'll remove the error checking for this case.

[goneall]

Fixed with #21